1. Field of the Invention
The invention relates to measuring solid particle number concentrations from engine or vehicle exhausts in real-time, and to a penetration and removal efficiency calibration unit for a volatile particle remover (VPR) in a solid particle counting system (SPCS).
2. Background Art
European Particle Measurement Program (PMP) proposed a draft regulation for measuring solid particle number emission in exhaust from light-duty diesel vehicles. As shown in FIG. 1, the measurement system consists of a pre-classifier 10, a hot particle diluter (PND1) 12, an evaporation unit (EU) 14, a cold particle diluter (PND2) 16, and a condensation particle counter (CPC) 18. The hot particle diluter (PND1) 12, evaporation unit (EU) 14, and cold particle diluter (PND2) 16 are referred to as the Volatile Particle Remover (VPR) 20. FIG. 1 shows a simplified schematic of the measurement system.
The VPR 20 dilutes diesel aerosol in PND1 12 and PND2 16. The EU 14 in the VPR 20 is operated at a high temperature (such as 300 to 400° C.) to evaporate volatile particles into gas phase. By following dilution from PND2 16 with room temperature dilution air, the aerosol is cooled down, and the volatile material concentration is reduced to the level to avoid the formation of the volatile particles. Thus, volatile particles are removed, and solid particles only move into the CPC 18. The concentration of the solid particles is measured in the CPC 18.
To have accurate measurement on solid particle concentration, PMP recommended that the solid particle penetration on the VPR 20 should be verified by mono-disperse solid particles at 30, 50, and 100 nm. The removal efficiency of the VPR 20 for volatile particles should be tested with mono-disperse C40 particles with 30 nm diameter. To measure penetrations for mono-disperse solid particle particles and removal efficiency for mono-disperse C40 particles on the VPR 20, mono-disperse particles need to be sent into the VPR with a CPC for the diluted concentration, and be sent into a CPC for the raw concentration. Equations 1 and 2 show the calculation for the penetration and removal efficiency:
                    P        =                                            C              Diluted                        *                          DR              1                        *                          DR              2                                            C            Upstream                                              (        1        )                                          E          Removal                =                  1          -          P                                    (        2        )            where, P is the penetration; ERemoval is the removal efficiency for C40 particles; CDiluted is the diluted concentration for mono-disperse particles (solid or C40 particles), CUpstream is the raw (upstream) concentration for mono-disperse particles; DR1 is the dilution ratio on the PND1; and, DR2 is the dilution ratio on the PND2. While the single size aerosol is connected to point A in FIG. 1, CDiluted is measured. While the single size aerosol is connected to point B in FIG. 1, CUpstream is measured.
The Differential Mobility Analyzer (DMA) is widely used to select the single size particles. The selected single size particle concentration is extremely sensitive to the inlet flow and the outlet flow from the DMA. With small change on those flows, large variation may be detected on the raw concentration particles. From equation 1, it is observed that the raw concentration CUpstream is assumed no change and as the same as the measured raw concentration while the mono-disperse aerosol is sent into the VPR. Any variation on the raw concentration for the mono-disperse particle causes the error on the penetration and removal efficiency. Thus, the concentration of the mono-disperse particles should be stable and kept unchanged.
Under most of circumstances, inlet flows for the VPR and a CPC are different. The system needs to be adjusted slightly to keep the outlet flow unchanged from the DMA. This flow needs to be monitored carefully to ensure the stable and constant concentration for the mono-disperse particles. Thus, the calibration test and experimental setups for solid particle penetration and removal efficiency with mono-disperse particles are time consuming, and require that the operator have good background and knowledge with aerosol science and particle instruments. This is unrealistic in the automobile industry since few operators have background for aerosol science and related technologies.